Thermal ink-jet printers operate by employing a plurality of resistor elements to expel droplets of ink through an associated plurality of nozzles. In particular, each resistor element, which is typically a pad of resistive material measuring about 50 .mu.m.times.50 .mu.m, is located in a chamber filled with ink supplied from an ink reservoir. A nozzle plate, comprising a plurality of nozzles, or openings, with each nozzle associated with a resistor element, defines part of the chamber. Upon energizing a particular resistor element, a droplet of ink is expelled through the nozzle toward the print medium, whether paper, fabric, or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. By proper selection of the signals, alphanumeric and other characters are formed on the print medium.
The tight clearances of the nozzles, typically 50 .mu.m diameter, require that the ink not clog them. Further, repeated firings of the resistor elements, which must withstand about 10 million firings over the life of the ink cartridge, can result in fouling of the resistor element. This phenomena is unique to thermal ink-jet printers and is known as kogation. The ink composition should be smear resistant and waterfast.
In copending application Ser. No. 07/309,516, (now U.S. Pat. No. 4,961,785) which has the same assignee as this application, inks for thermal ink-jet printers which contain about 15% to about 50% formamide are shown to have improved smear resistance.
Another problem encountered in this field is known as feathering. This is caused by the tendency of the ink to spread from the spot where it was placed by the printer. It is the result of the physico-chemical interactions between the wet ink and the paper.
Yet another problem is an unwanted mixing, in the case of multicolor printing, of adjacent colored areas. This is termed color bleed and is the result of the mixing of two wet inks. Both feathering and color bleed result in degraded edge definition.
Inks are known which possess one or more desirable properties. However, few ink compositions are known that possess all the desirable properties, since an improvement in one property often results in the degradation of another property. Thus, many inks used commercially represent a compromise in an attempt to achieve an ink evidencing at least an adequate response in each of the foregoing considerations.
Accordingly, investigations continue into developing ink formulations which have improved properties and which do not elevate one property at the expense of the others.